The combination of a Bronsted polybase, defined as a polymer possessing proton acceptor centers, with a metal halide (MX) inorganic salt in the absence of a solvent has been very thoroughly investigated because of the ionic conduction properties of the resultant product. EP-A-13 199, for example, discloses macromolecular materials capable of ionic conduction. These materials comprise a solid solution of an alkali metal salt dissolved within a basic polymer, comprising heteroatoms selected from among oxygen, nitrogen, sulphur and phosphorous. The best conductivity was obtained with a complex of polyoxyethylene and lithium salt. Nevertheless, these materials are not entirely satisfactory for all purposes. A number of investigators have attempted to remedy the deficiencies of these materials without, however, achieving entirely satisfactory results.
It is known that conduction takes place almost exclusively when a polymer is in the elastomeric amorphous phase, such that the ions and the segments of the polymer are readily mobile. Moreover, polyether and polyimine homopolymers have a tendency to crystallize very easily. The introduction of certain macromolecular materials has been proposed to prevent such crystallization. EP-A-119 912, for example, describes macromolecules comprising copolymers of ethylene oxide with a second functional group comprising a cyclic etheroxide. Polymers are thus obtained which are less likely to crystallize than homopolymers having linear chains. On the other hand, however, the energy of solvation of the salt by the polymer is reduced due to the reduction in the number of basic centers carried by the polymer which are capable of dissociating the salt. There is thus a consequent reduction in the ionic conductivity of the material.
If crystallization of the polymer is to be prevented, the same is true of the vitrification of the polymer. Thus, the second criterion for selecting the polymer is that the material have a vitreous transition temperature which is as low as possible. This preserves the viscoelastic properties needed for the movement of the various chains and which, moreover, favor the proper use of the material. The material must be capable of being used in thin layers and must therefore be sufficiently flexible. In order to increase this flexibility, it is possible to "carry" the material to a higher temperature. Although temperatures in the vicinity of 80.degree.-100.degree. C. may be used for applications such as electrochemical generators, the same is not true for the use of this material in forming electrochromic panes as described below. Such panes which must be capable of operating at ambient temperature and even at temperatures below -10.degree. C.
Further, analysis of the mechanism of conduction has shown that the anionic transport number is often equivalent to the cationic transport number, even with ClO.sub.4.sup.-, SCN.sup.- and CF.sub.3 SO.sub.3.sup.- anions, the volume of which is, however, much greater than the volume of Li.sup.+, Na.sup.+ or K.sup.+ cations. Moreover, the conduction of the cation fulfils an important role in various mechanisms, such as electrochromism. EP-A-213 985 discloses salts, the anion of which is present in the form of a polyether chain, wherein the end of the chain carries an anionic function. The length of the chain limits the mobility of the anion. In the same sense, it has also been proposed to graft the anions onto the polymeric chain or to associate them with this chain with hydrogen bonds.
Generally speaking, the ionic conductivities obtained at ambient temperatures are relatively low, for example, less than 10.sup.-5 .OMEGA..sup.-1 cm.sup.-1, which excludes the use of such materials for many practical applications.
Furthermore, the presence of water should systematically be avoided in these mixtures since the degree of conduction by protons and hydroxyls has sometimes been found to be greater than that attributable to the metallic cation. Moreover, water is highly injurious to the electrochemical stability of the electrolyte. It is necessary to operate within a narrow water concentration range, since gaseous hydrogen is otherwise formed and small bubbles appear. Moreover, water generally leads to accelerated ageing and, in addition, it may dissolve the thin layers of material in contact with the electrolyte.
Experiments have also been performed with materials which conduct alkali metals. Such materials are obtained by mixing an ionomer (i), produced by neutralization of a Bronsted polyacid with an alkali metal hydroxide (MOH), with a polar solvent (ii) capable of dissolving or at least plasticizing the ionomer (i).
It has recently been demonstrated that ionomers, such as sodium poly(styrene sulphonate) became polyelectrolytes conductive of the sodium ion, only when plasticized by poly(ethylene glycol) (PEG) having a low molecular weight (L. C. Hardy and D. F. Shriver, Journal of the American Chemical Society, 107, 3823 (1985)). Nevertheless, the conductivity value at ambient temperature remains lower than 10.sup.-6 .OMEGA..sup.-1 cm.sup.-1.
FR-2 597 491 describes a method of manufacturing colloids of perfluorinated ionomers by dissolving, in a suitable polar solvent, salts of NAFION.RTM. (a registered trademark of Du Pont de Nemours), a fluorinated polymer carrying -SO.sub.3 M ion exchanger groups, where M.sup.+ is a metallic cation or a more complex cationic entity. The ionic conductivity of such colloids (sols or gels) may reach 5.times.10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 at ambient temperature. However, the products used have a high initial cost, which limits the use of such materials.
Additionally, a number of references have described the materials produced by combining an ionomer (i) as defined above and a Bronsted polybase (iii) of a polymer having proton acceptor centers as described above. The hydrogen bonding between acid functions of the first polymer and basic functions of the second polymer may lead to the formation of an intermolecular complex, following which the polymers are placed in a common solvent for mixing.
It is known, for example, that the addition of aqueous solutions of a polyether of a polyoxyethylene and of polyacrylic acid leads to the formation of a precipitate possessing an amorphous character and a rubbery appearance. The vitreous transition temperature of this precipitate is intermediate between that of polyoxyethylene and polyacrylic acid. In the complex thus formed, the two polymers are very highly imbricated due to the very large number of hydrogen bridges which are established. Thus, numerous polymer complexes, which give rise to proton acceptor proton-polymers, are likely to be obtained, provided that the forces of interaction between them are greater than the sum of the energies of solvation in the initial solutions.
A different result occurs if, before the mixing of the polymers, a certain number of the acid functions are neutralized by a base such as lithium hydroxide, sodium hydroxide or potassium hydroxide. The complexing between the two polymers can then no longer take place except through the intermediary of the non-neutralized acid functions, with correlation lengths that are shorter to the extent to which repulsive interactions appear between the ionized --COO.sup.- groups and the oxygens of the polyether. This partial neutralization therefore enables the desired cations to be introduced into the body of the macromolecular material, thus giving it the characteristic of an ionic conductor. Furthermore, the separation of the chains due to these repulsive interactions leads to an expansion of the polymers, which are formed into a gel that can easily be applied in thin layers.
As described in an article by M. Tsuchida et al., Solid State Ionics 116, 227-233 (1983), beyond a certain limiting neutralization rate (.alpha..sub.m) the complex no longer forms. The different phases separate unless quantities of solvents incompatible with the envisaged uses are employed. Partial neutralization should therefore be understood to mean a neutralization rate (.alpha.) such that O &lt;.alpha.&lt;.alpha..sub.m.
Adequate conduction is possible only in the presence of a basic solvent, which enables the alkaline cations to be kept away from the influence of the --COO.sup.- groups of the acid polymer. In the absence of such a basic solvent, the pairs of ions are so strongly associated that the cation is in practice no longer mobile. The resultant material is thus a fairly poor ionic conductor.
When water is used as a basic solvent, a very high ionic conductivity is obtained, for example of the order of 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 at 20.degree. C. This value is 100 to 1,000 times greater than that obtainable with macromolecular materials having a known ionic conductivity. Nevertheless, this high conductivity is probably partly due to the dissociation of the water arising from the solvent and the protons of the acid polymer. On the other hand, as discussed above, the presence of water is not always desirable in electrochemical devices, particularly in those devices which use reversible electrodes in contact with an aqueous solution, such as devices formed partly of electrochromic materials.
In such cases, it is preferable to use as the basic solvent a polar organic solvent, chosen as a function of the nature of the acid and basic polymers, (the basic solvent should not cause the mixture of polymers to segregate), of the acidity of the acid polymer, of the neutralization rate (.alpha.) and of the nature of the cation M.sup.+. It is known, for example, that the interaction of the COO- groups with M.sup.+ decreases in the series LI.sup.+, Na.sup.+, K.sup.+, and of its ranges of thermal and electrochemical stability, although on these two latter points the majority of the polar organic solvents possess properties better than those of water.